1. Field of the Invention
The invention relates to a method for sensing a potential threat to an object from an entity which passes radio links from navigation satellite transmitters to receiving stations.
That method is distinguished in that, without requiring any three-dimensional alignment, it can quickly monitor the entry of an entity, from which a threat could originate, from a potentially threatened, also mobile object, from the hemispherical airspace. This is achieved on the basis of the passive-radar principle using the coherent signal structures which are emitted from the available navigation satellite transmitters, are highly constant over time and are known in the same way as the orbit data for non-geostationary satellites. Their current received signals are evaluated for irregularities in particular such as secondary signals that are dependent on back scatter in comparison to the received signals to be expected, with secondary signals which are dependent on back scatter such as these being caused by an entity flying into the transmission paths from the satellite transmitters to the receiver of the object. The characteristics, which are similar to radar, of the satellite frequencies, in terms of the fact that they propagate largely in straight lines, and their modulation forms (spread spectrum and noise periods) which require correlation assist their suitability for monitoring the airspace based on the principle of multistatic radar with a passive secondary transmitter. Details can be found in that prior publication.
2. Discussion of the Prior Art
Because of the already available navigation satellite systems, the method of this generic type, as described in U.S. Pat. No. 5,424,744, which is commonly assigned to the assignee of this application, and the entire disclosure of which is incorporated by reference, can be used throughout the world without any investment requirements for the transmitters. It operates passively and therefore without posing any threat itself. However, once a supposed entity has been detected for the first time, its continuous observation is quite time-consuming in order to obtain information in particular about its velocity (based on frequency measurements), range (based on propagation time measurements) and position (relative to the location of that satellite whose transmitter is currently being received), with this information being that which is required to decide on and then to initiate any defensive reaction which may become necessary against the threatening entity. This can critically restrict the time interval which remains for a defensive reaction.
Against the background of these circumstances, the present invention is based on the technical problem of significantly improving the method mentioned initially, which has been proven per se, without any need to have to develop new technologies, or else just introduce them, that is to say using technological means which are already available, in terms of its efficiency with regard to the capabilities to make statements about the current threat scenarios, in order to improve the reaction times.